1. Field of the Invention
The invention concerns a method for detecting trace quantities of gases or gaseous impurities in a gaseous medium or in a monitored atmosphere by measuring the electrical conductivity of a sensor fabricated from a metal oxide selected from the group comprising SnO.sub.2 and NiO, in which method said sensor is placed in a detector device in which it is heated and exposed to and/or supplied with the gaseous medium to be monitored. The invention also concerns a metal oxide sensor of this type and a device for implementing the method and comprising the sensor.
2. Description of the Prior Art
The semiconducting properties of certain metal oxides are well known. Their electrical conductivity varies as a function of temperature and in the presence of certain gases. These properties have been used for the detection of trace quantities of gases or gaseous impurities. The oxides used for this purpose are generally tin oxide SnO.sub.2 and nickel oxide NiO. They are used in the crystalline state or in the form of a layer deposited on a ceramic support or in the form of pellets of sintered powder. Applications of this kind are described, inter alia, in U.S. Pat. Nos. 3,635,756, 3,900,815 and 4,039,941.
A great number of gases affect the conductivity of these oxides, however, as a result of which they are not very reliable when used as detector sensors, for want of a specific or selective action.
The invention, to be disclosed in detail later, is based on the discovery of a treatment which confers on the aforementioned semiconductor oxides electrical conductivity characteristics of an entirely novel kind permitting the selective detection of specific gases or groups of gases.
As a general rule, semiconductor oxides have an electrical conductivity .sigma. which varies with temperature and with the nature and pressure of the gases constituting its surrounding environment:
.sigma. increases exponentially with temperature, PA1 the sense in which .sigma. changes depends on the nature of the gas, PA1 the amplitude of the change depends on the partial pressure of the gases and the composition of the gaseous phase. PA1 1. The peak moves in the direction of increasing temperature and reduces the intensity to the point of disappearing completely if the sensor is raised to too high a temperature, of the order of 550.degree. C. PA1 2. The peak occurs only if the sensor has previously been exposed to SO.sub.2 or H.sub.2 S. This treatment is irreversible unless the sensor is treated by saturating it for several hours, for example.
Chemisorption of a gas by a semiconductor oxide involves a transfer of electrons between the gas and the oxide. Depending on whether the gas g has a higher or lower affinity for electrons than the oxide s, there is a transfer of electrons from the solid to the gas (s.sup.+ -g.sup.-) or from the gas to the solid (s.sup.- -g.sup.+), respectively. This transfer contributes to an increase or decrease in the number of free majority carriers in the solid. The sense of the variation is defined by the type of bond (g.sup.+ -s.sup.- or g.sup.- -s.sup.+) and by the nature of the majority carriers, that is to say electrons in the case of n type semiconductors and holes in the case of p type semiconductors.
It is known that the electrical conductance G of a semiconductor is substantially proportional to the number of majority carriers. Consequently, the presence of a gas varies the conductance of the semiconductor according to whether the gas adsorbed takes up electrons from or donates electrons to the semiconductor.
It is then said to be either an electron acceptor or donor, respectively.
If the gas is an acceptor and if the semiconductor metal oxide is of type n, the gas takes an electron from the solid, the conductance of which diminishes. If the gas is an acceptor and the semiconductor metal oxide is of type p, the gas takes an electron from the solid, the conductance of which increases.
If the gas is a donor, the senses of the electrical variations observed will be the inverse of those just defined. In other words, for a specific gas there is always observed a variation in conductivity according to the concentration of the gas. It is therefore only necessary to measure such variation in order to detect the presence of this gas. The sensors currently used and based on this principle are relatively sensitive in practice, but they have the major disadvantage of offering an electrical response which does not provide for selective or specific detection of a given gas in a mixture.
The objective of the invention is to propose a new method, sensor and device for selective detection of gases with a very high degree of sensitivity.